High intensity discharge (HID) lamps are used in a variety of general purpose and specialty lighting applications. Included in this category of lamps are high-pressure sodium lamps, mercury vapor lamps, rare gas discharge lamps, and metal halide lamps. One specialized application for HID lamps is video projection including projection-type televisions. These lamps are generally expensive, costing on the order of hundreds of dollars, and consequently are sold to customers with a warranty for a guaranteed minimum operating life. Replacements for lamps that are returned to the manufacturer because they did not meet their guaranteed lifetime can represent a significant cost. At present, there is no reliable means for the manufacturer to evaluate the propriety of such warranty claims. For example, production date marking is insufficient to determine the actual operating life of a lamp because of unknown storage times at the wholesaler, retailer, or end user.
Some industries have developed visual-change timers to indicate an end-of-life condition for their products. For example, organic-based, color-change timers have been widely used in the containers or wrappings of perishable agricultural products to serve as a warning that the product is past its expiration date. Commercially available color timers used in connection with foodstuffs however are limited to functioning at temperatures less than about 60° C. This is much lower than the operating temperatures generated in an operating HID lamp. Moreover, an HID lamp will operate for hundreds or, even more likely, thousands of hours at these high temperatures. Hence, a color-change timer for an HID lamp would have to function for long periods of time at conditions that would decompose most organic materials. It would therefore be advantageous to provide a high-temperature, visual-change timer that could be used with HID lamps to provide an indication of the cumulative operating time or an end-of-life condition.